Add length constraint for workflow tasks

Now with empty tasks list template will fail
heat temlate validation.

Change-Id: Ibe7e031771687aadf15f5bc2e57ab7a294f48e21
Closes-Bug: #1558514

heat/engine/resources/openstack/mistral/workflow.py

@@ -331,7 +331,8 @@ class Workflow(signal_responder.SignalResponder,
                 },
             ),
             required=True,
-            update_allowed=True
+            update_allowed=True,
+            constraints=[constraints.Length(min=1)]
         )
     }